Hydrodynamic effects an galvanising of high strength steels

A modification of the laboratory gaivanisation equipment was made to reproduce the zinc movement along the strip as it is the case industrially during hot dipping. The galvanisation tests in baths containing between 0.24wt% and 0.28wt% of aluminium show an improvement of the zinc wettability on high strength steel to the same level as on industrial lines. The same kind of defects (small bare spots) can also be observed when high strength steels with large amounts of alloying elements are galvanised. Comparable inhibition layer and coating adhesion are however obtained with or without a bath stirring. For galvannealing baths where the aluminium content is smaller (0.135 % Al-eff), the aluminium depletion obtained at the steel/liquid zinc interface is more important in the non-stirred baths. That depletion of aluminium is no more compensated by the long dipping time (3s) and a thinner inhibition layer is formed on the steel surface with static baths. The thicker inhibition layer, more representative of industrial conditions, induces an increase of the galvannealing temperature to completely transform the coating.